Effect of nanoparticle deposition on rewetting temperature and quench velocity in experiments with stainless steel rodlets and nanofluids

Quenching of small stainless steel rods in pure water and nanofluids with alumina and diamond nanoparticles at low concentrations (0.1 vol%) was investigated experimentally. The rods were heated to an initial temperature of ~1000 C and then plunged into the test fluid. The results show that the quenching behavior of the nanofluids is nearly identical to that of pure water. However, due to nanofluids boiling during the quenching process, some nanoparticles deposit on the surface of the rod, which results in much higher quenching rate in subsequent tests with the same rod. It is likely that particle deposition destabilizes the film-boiling vapor film at high temperature, thus causing the quenching process to accelerate, as evident from the values of the quench front speed measured by means of a high-speed camera. The acceleration strongly depends on the nanoparticle material used, i.e., the alumina nanoparticles on the surface significantly improve the quenching, while the diamond nanoparticles do not. The possible mechanisms responsible for the quench front acceleration are discussed. It is found that the traditional concept of conduction-controlled quenching cannot explain the acceleration provided by the nanoparticle layer on the surface. NOMENCLATURE Bi Biot number (=c/k) c Specific heat (J/kg-K) f Correction factor for wettability h Heat transfer coefficient (W/m-K) k Thermal conductivity (W/m-K) q Heat flux (W/m) R Radius of rodlet (m) t Time (s) T Temperature (C) Tc Critical temperature of water (C) Tint Interface temperature (C) TMAX Maximum temperature of liquid phase(C) TMFB Minimum film boiling temperature (C) Trew Rewetting temperature (C) Trod Temperature of rod specimen (C) T* Dimensionless temperature, ) ( ) ( f rew f rod T T T T   u Propagation velocity of quench front (m/sec) vol% Volume percent Tsat Wall superheat (C) Greek Letter  Density (kg/m)  Thermal effusivity (W/m-K) Subscripts f Fluid sat Saturated w Wall INTRODUCTION A number of recent investigations on boiling of nanofluids showed that such engineered fluids can effectively delay departure from nucleate boiling (DNB) with respect to pure fluids (You et al., 2003; Vassallo et al., 2004). It was found that the DNB heat flux enhancement is closely related to nanoparticle deposition, which may change the heater surface roughness significantly (Bang and Chang, 2005; Kim et al., 2006). Moreover, the deposition of oxide nanoparticles like alumina and titania significantly enhances the affinity, or wettability, of the liquid to the surface (Kim et al., 2007). These surface changes alter the boiling heat transfer characteristics, e.g., they increase the value of the critical heat flux. Park et al. (2004) performed quenching experiments of a copper sphere in alumina nanofluids to investigate the effect of the nanoparticles on film boiling heat transfer. Their results showed that the film-boiling heat transfer rate in nanofluids was somewhat lower than in pure water. However, they observed